The next generation of optical memory is seeking to provide advances in capacity, data density, and data security. One such technology is the multi-layer CD-ROM structure. The multi-layer structure provides a data storage capacity multiplier according to the number of layers. The fabrication process of multi-layer structure requires multiple steps of data transfer and layer bonding process. Due to the requirements of addressing each layer without cross talk, the separation between layers can be as much as 30 microns. This translates into a requirement for an optical system with long focal length that can address the widely separated layers. However, in order to obtain high storage density in each layer, a smaller laser spot size and higher numeric aperture (“NA”) objective are required. In turn, the higher NA yields a smaller depth of focus (“DOF”). This combination of effects leads to the limit of a small number of layers that are possible with this memory structure thus reducing its overall capacity. In order to increase the overall capacity, vertical bit-to-bit separation needs to be reduced significantly.
Another multi-layer approach for an optical memory structure uses a stack of metal thin films, with up to eight layers, each layer 5 to 30 nanometers (“nm”) thick. In this structure, up to 256 shades of gray intensity represent eight bits of information. Due to the strong optical reflection characteristic properties of metals and the difficulty of controlling metal thin film deposition precision in one nanometer (nm) or less, only a small number of layers are realistic. Otherwise, the gray scale quantum step would be too small to obtain a good signal-to-noise (“S/N”) ratio margin and the bit error rate (“BER”) would be too high to meet the requirement of a computing memory. This approach is only suitable for producing a gray scale micro-picture for direct eye viewing (where a high BER is tolerable), instead of computing data applications.
Other optical memory devices attempting to use color as memory utilizing “colorants” such as color matter, color ink, colored plastic films, or color photographic system are known in the art. The detection system uses color detectors (strictly speaking, bandpass detectors) to detect the individual passband intensity and a complex computation process to extract data. For each colorant, there has to be a passband corresponding to it. A combination of different colorants in a cluster is called a “super-pixel” (which is several square micron in size) and is used as an optical access spot unit. The shortcomings of this invention include the aging of the memory colorant, the small practical number of colorants usable due to the broad optical bandwidth of the colorant and the difficulties in fabrication of the super-pixel with small size.
Consequently, a significant need exists for a high density optical storage device that stores multiple of bits of data per pixel at an accuracy level suitable for digital processing.